This invention relates to methods for obtaining antibodies and assays for using such antibodies. More specifically, the invention relates to methods of obtaining antibodies which specifically bind to naturally occurring forms of PrPSc.
Prions are infectious pathogens that cause central nervous system spongiform encephalopathies in humans and animals. Prions are distinct from bacteria, viruses and viroids. The predominant hypothesis at present is that no nucleic acid component is necessary for infectivity of prion protein. Further, a prion which infects one species of animal (e.g., a human) will not infect another (e.g., a mouse).
A major step in the study of prions and the diseases that they cause was the discovery and purification of a protein designated prion protein (xe2x80x9cPrPxe2x80x9d) [Bolton et al., Science 218:1309-11 (1982); Prusiner, et al., Biochemistry 21:6942-50 (1982); McKinley, et al., Cell 35:57-62 (1983)]. Complete prion protein-encoding genes have since been cloned, sequenced and expressed in transgenic animals. PrPc is encoded by a single-copy host gene [Basler, et al., Cell 46:417-28 (1986)] and is normally found at the outer surface of neurons. Prion diseases are accompanied by the conversion of PrPc into a modified form called PrPSc. However, the actual biological or physiological function of PrPc is not known.
The scrapie isoform of the prion protein (PrPSc ) is necessary for both the transmission and pathogenesis of the transmissible neurodegenerative diseases of animals and humans. See Prusiner, S. B., xe2x80x9cMolecular biology of prion disease,xe2x80x9d Science 252:1515-1522 (1991). The most common prion diseases of animals are scrapie of sheep and goats and bovine spongiform encephalopathy (BSE) of cattle [Wilesmith, J. and Wells, Microbiol. Immunol. 172:21-38 (1991)]. Four prion diseases of humans have been identified: (1) kuru, (2) Creutzfeldt-Jakob Disease (CJD), (3) Gerstmann-Strassler-Scheinker Disease (GSS), and (4) fatal familial insomnia (FFI) [Gajdusek, D.C., Science 197:943-960 (1977); Medori et al., N. Engl. J. Med. 326:444-449 (1992)]. The presentation of human prion diseases as sporadic, genetic and infectious illnesses initially posed a conundrum which has been explained by the cellular genetic origin of PrP.
Most CJD cases are sporadic, but about 10-15% are inherited as autosomal dominant disorders that are caused by mutations in the human PrP gene [Hsiao et al., Neurology 40:1820-1827 (1990); Goldfarb et al., Science 258:806-808 (1992); Kitamoto et al., Proc. R. Soc. Lond. (In press) (1994)]. Iatrogenic CJD has been caused by human growth hormone derived from cadaveric pituitaries as well as dura mater grafts [Brown et al., Lancet 340:24-27 (1992)]. Despite numerous attempts to link CJD to an infectious source such as the consumption of scrapie infected sheep meat, none has been identified to date [Harries-Jones et al., J. Neurol. Neurosurg. Psychiatry 51:1113-1119 (1988)] except in cases of iatrogenically induced disease. On the other hand, kuru, which for many decades devastated the Fore and neighboring tribes of the New Guinea highlands, is believed to have been spread by infection during ritualistic cannibalism [Alpers, M. P., Slow Transmissible Diseases of the Nervous System, Vol. 1, S. B. Prusiner and W. J. Hadlow, eds. (New York: Academic Press), pp. 66-90 (1979)].
The initial transmission of CJD to experimental primates has a rich history beginning with William Hadlow""s recognition of the similarity between kuru and scrapie. In 1959, Hadlow suggested that extracts prepared from patients dying of kuru be inoculated into non-human primates and that the animals be observed for disease that was predicted to occur after a prolonged incubation period [Hadlow, W. J., Lancet 2:289-290 (1959)]. Seven years later, Gajdusek, Gibbs and Alpers demonstrated the transmissibility of kuru to chimpanzees after incubation periods ranging form 18 to 21 months [Gajdusek et al., Nature 209:794-796 (1966)]. The similarity of the neuropathology of kuru with that of CJD [Klatzo et al., Lab Invest. 8:799-847 (1959)] prompted similar experiments with chimpanzees and transmissions of disease were reported in 1968 [Gibbs, Jr. et al., Science 161:388-389 (1968)]. Over the last 25 years, about 300 cases of CJD, kuru and GSS have been transmitted to a variety of apes and monkeys.
The expense, scarcity and often perceived inhumanity of such experiments have restricted this work and thus limited the accumulation of knowledge. While the most reliable transmission data has been said to emanate from studies using non-human primates, some cases of human prion disease have been transmitted to rodents but apparently with less regularity [Gibbs, Jr. et al., Slow Transmissible Diseases of the Nervous System, Vol. 2, S. B. Prusiner and W. J. Hadlow, eds. (New York: Academic Press), pp. 87-110 (1979); Tateishi, et al., Prion Diseases of Humans and Animals, Prusiner, et al., eds. (London: Ellis Horwood), pp. 129-134 (1992)].
The infrequent transmission of human prion disease to rodents has been cited as an example of the xe2x80x9cspecies barrierxe2x80x9d first described by Pattison in his studies of passaging the scrapie agent between sheep and rodents [Pattison, I. H., NINDB Monograph 2, D. C. Gajdusek, C. J. Gibbs Jr. and M. P. Alpers, eds. (Washington, D.C.: U.S. Government Printing), pp. 249-257 (1965)]. In those investigations, the initial passage of prions from one species to another was associated with a prolonged incubation time with only a few animals developing illness. Subsequent passage in the same species was characterized by all the animals becoming ill after greatly shortened incubation times.
The molecular basis for the species barrier between Syrian hamster (SHa) and mouse was shown to reside in the sequence of the PrP gene using transgenic (Tg) mice [Scott, et al., Cell 59:847-857 (1989)]. SHaPrP differs from MoPrP at 16 positions out of 254 amino acid residues [Basler, et al., Cell 46:417-428 (1986); Locht, et al., Proc. Natl. Acad. Sci. USA 83:6372-6376 (1986)]. Tg(SHaPrP) mice expressing SHaPrP had abbreviated incubation times when inoculated with SHa prions. When similar studies were performed with mice expressing the human, or ovine PrP transgenes, the species barrier was not abrogated, i.e., the percentage of animals which became infected were unacceptably low and the incubation times were unacceptably long. Thus, it has not been possible, for example in the case of human prions, to use transgenic animals (such as mice containing a PrP gene of another species) to reliably test a sample to determine if that sample is infected with prions. The seriousness of the health risk resulting from the lack of such a test is exemplified below.
More than 45 young adults previously treated with HGH derived from human pituitaries have developed CJD [Koch, et al., N. Engl. J. Med. 313:731-733 (1985); Brown, et al., Lancet 340:24-27 (1992); Fradkin, et al., JAMA 265:880-884 (1991); Buchanan, et al., Br. Med. J. 302:824-828 (1991)]. Fortunately, recombinant HGH is now used, although the seemingly remote possibility has been raised that increased expression of wtPrPc stimulated by high HGH might induce prion disease [Lasmezas, et al., Biochem. Biophys. Res. Commun. 196:1163-1169 (1993)]. That the HGH prepared from pituitaries was contaminated with prions is supported by the transmission of prion disease to a monkey 66 months after inoculation with a suspect lot of HGH [Gibbs, Jr., et al., N. Engl. J. Med. 328:358-359 (1993)]. The long incubation times associated with prion diseases will not reveal the full extent of iatrogenic CJD for decades in thousands of people treated with HGH worldwide. Iatrogenic CJD also appears to have developed in four infertile women treated with contaminated human pituitary-derived gonadotrophin hormone [Healy, et al., Br. J. Med. 307:517-518 (1993); Cochius, et al., Aust. N.Z. J. Med. 20:592-93 (1990); Cochius, et al., J. Neurol. Neurosurg. Psychiatry 55:1094-1095 (1992)] as well as at least 11 patients receiving dura mater grafts [Nisbet, et al., J. Am. Med. Assoc. 261:1118 (1989); Thadani, et al., J. Neurosurg. 69:766-769 (1988); Willison, et al., J. Neurosurg. Psychiatric 54:940 (1991); Brown, et al., Lancet 340:24-27 (1992)]. These cases of iatrogenic CJD underscore the need for screening pharmaceuticals that might possibly be contaminated with prions.
Recently, two doctors in France were charged with involuntary manslaughter of a child who had been treated with growth hormones extracted from corpses. The child developed Creutzfeldt-Jakob Disease. (See New Scientist, Jul. 31, 1993, page 4). According to the Pasteur Institute, since 1989 there have been 24 reported cases of CJD in young people who were treated with human growth hormone between 1983 and mid-1985. Fifteen of these children have died. It now appears as though hundreds of children in France have been treated with growth hormone extracted from dead bodies at the risk of developing CJD (see New Scientist, Nov. 20, 1993, page 10.) Prior attempts to create PrP monoclonal antibodies have been unsuccessful (see Barry and Prusiner, J. of Infectious Diseases Vol. 154, No. 3, Pages 518-521 (1986). Thus there is a need for an assay to detect compounds which result in disease. Specifically, there is a need for a convenient, cost-effective assay for testing sample materials for the presence of prions which cause CJD. The present invention offers such an assay.
Antibodies of the invention will specifically bind to a native prion protein (i.e., native PrPSc) in situ with a high degree of binding affinity. The antibodies can be placed on a substrate and used for assaying a sample to determine if the sample contains a pathogenic form of a prion protein. The antibodies are characterized by one or more of the following features (1) an ability to neutralize infectious prions, (2) will bind to prion proteins (PrPSc) in situ i.e., will bind to naturally occurring forms of a prion protein in a cell culture or in vivo and without the need to treat (e.g., denature) the prion protein, and (3) will bind to a high percentage of the PrPSc form (i.e. disease form) of prion protein in a composition e.g., will bind to 50% or more of the PrPSc form of the prion proteins. Preferred antibodies are further characterized by an ability to (4) bind to a prion protein of only a specific species of mammals e.g., bind to human prion protein and not prion protein of other mammals.
An important object is to provide antibodies which bind to native prion protein (PrPSc).
Another object is to provide antibodies which specifically bind to epitopes of prion proteins (PrPSc) of a specific species of animal and not to the prion protein (PrPSc) of other species of animals.
Another object is to provide monoclonal antibodies which specifically bind to prion proteins (PrPSc) associated with disease, (e.g., human PrPSc) which antibodies do not bind to denatured PrP proteins not associated with disease (e.g., human PrPc).
Still another object is to provide specific methodology to allow others to generate a wide range of specific antibodies characterized by their ability to bind one or more types of prion proteins from one or more species of animals.
Another object of the invention is to provide an assay for the detection of PrPSc forms of PrP proteins.
Another object of the invention is to provide an assay which can specifically differentiate prion protein (PrPSc) associated with disease from PrPSc not associated with disease.
Another object is to detect prions which specifically bind to native PrPSc of a specific species such as a human, cow, sheep, pig, dog, cat or chicken.
An advantage of the invention is that it provides a fast, efficient cost effective assay for detecting the presence of native PrPSc in a sample.
A specific advantage is that the assay can be used as a screen for the presence of prions (i.e., PrPSc) in products such as pharmaceuticals (derived from natural sources) food, cosmetics or any material which might contain such prions and thereby provide further assurances as to the safety of such products.
Another advantage is that the antibodies which can be used with a protease which denatures PrPc thereby providing for a means of differentiating between infectious (PrPSc) and non-infectious forms (PrPSc) of prions.
Yet another advantage of the invention is that antibodies of the invention are characterized by their ability to neutralize the infectivity of naturally occurring prions e.g., neutralize PrPSc.
Another advantage is that antibodies of the invention will bind to (PrPSc) prion proteins in situ, i.e., will bind to naturally occurring (PrPSc) prions in their natural state in a cell culture or in vivo without requiring that the prion proteins be particularly treated, isolated or denatured.
Another advantage is that the prion proteins of the invention will bind to a relatively high percentage of the infectious form of the prion protein (e.g., PrPSc)xe2x80x94for example bind to 50% or more of the PrPSc form of prion proteins in a composition.
An important feature of the invention is that the methodology makes it possible to create a wide variety of different prion protein antibodies with the same or individually engineered features which features may make the antibody particularly suitable for uses such as (1) prion neutralization to purify a product, (2) the extraction of prion proteins and (3) therapies.
A feature of the invention is that it uses phage display libraries in the creation of the antibodies.
Another feature of the invention is that the phage are genetically engineered to express a specific binding protein of an antibody on their surface.
These and other objects, advantages, and features of the invention will become apparent to those persons skilled in the art upon reading the details of the chimeric gene, assay method, and transgenic mouse as more fully described below.